Why Are My Namespaces Invisible in Visual Studio 2010 After Adding System.Web?

Visual Studio 2010 Namespace Visibility Conundrum

In Visual Studio 2010, a puzzling issue has surfaced where namespaces in a DLL project suddenly become invisible to a referencing executable project (Sandbox). This issue arises after adding System.Web and seemingly benign code to the DLL project.

Troubleshooting

Initial attempts to resolve the issue by removing and re-adding the DLL reference in the Sandbox project provide temporary relief, but the problem persists when attempting to build the solution. The object browser within Visual Studio confirms the presence of the namespace in the DLL project reference.

Clues and Resolutions

After carefully examining the changes, a specific line of code was identified as the apparent culprit:

string url = "http://maps.google.com?q=" + HttpUtility.UrlEncode(address);

Comment out this line eliminates the namespace errors and allows the project to build successfully.

The Bug Revealed

The issue stems from the default selection of the .NET Framework 4 Client Profile in the Target Framework settings for the affected projects. This profile excludes System.Web, which is required for the code in question.

The Fix

The solution is to manually change the Target Framework to ".NET Framework 4" in the Project Properties under the Application tab. This ensures that the full .NET Framework 4 is used, which includes System.Web.

Future Enhancements

In Visual Studio 2012, this issue was resolved by eliminating the Client Profile as the default selection for new projects. With the retirement of the Client Profile in .NET 4.5, this issue has been permanently addressed.

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